I downloaded gentoo-ppc-1.2.iso which schould be autobotable image, but it's not (at least on my Wallstreet Powerbook). I can't start installer with it - is there any solution for such behaviour? (I tired to press C-key, OPTION+C etc. while booting - there's no use).

The installer cd is in fact _not_ oldworld bootable (thank Apple computer and them not allowing anyone to use the cdrom boot driver)

You ahve to install bootx in MacOS and boot from there, which you will also have to do after yoe are installed. The wallstreet will not work properly with bootloaders such as quik, etc. (I own a wallstreet and have fought this battle many times)